Omnirange beacon antenna



June 9, 1964 E. G. PARKER 3,136,996

OMNIRANGE BEACON ANTENNA Filed Oct. l5, 1960 4 Sheets-Sheet 1 LOWF/QEQUEA/C Y BAA/D l o F, Fa F/efqvaEA/cy 98o Mc nes/wc RF @EN F/ 5ATTo/eA/EY June 9, 1964 E. G. PARKER 3,136,996

OMNIRANGE BEACON ANTENNA Filed Oct. 13, 1960 4 Sheecs-Sheecl 2 INVENTOR.

NVQ/VM@ ATTORNEY June 9, 1964 Filed Oct. 13, 1960 E. G. PARKER OMNIRANGEBEACON ANTENNA 4 Sheets-Sheet 5 INV EN TOR.

' ATTORNEY June 9, 1964 E. G. PARKER 3,136,996

OMNIRANGE BEACON ANTENNA Filed Oct. 15, 1960 4 Sheets-Sheet 4 q/l /l Ilm/l/l/l V41/ lll INVEN TOR. EKA/E57' G. PAR/ 67? BY yA/VL ATTORNEY UnitedStates Patent O 3,136,996 OMNIRANGE BEACUN ANTENNA Ernest G. Parker,Morristown, NJ., assigner to International Telephone and TeiegraphCorporation, Nutley, NJ., a corporation of Maryland Filed st. 113,196i), Ser. No. 62,437 22 Claims. (Ci. 343-106) My invention relates toparasitic elements for use with antennas for creating a radiationpattern of the antenna. More specifically, the invention relates tomeans for selectively controlling the parasitic elements.

In the past, a number of different techniques have been used to createan antenna which produces a space radiation pattern which rotatesthrough space according to a predetermined pattern. In many cases, ithas been found desirable to create a space radiation pattern whichvaries systematically with the azimuth from the antenna as measured froma reference direction. Antennas of the type used to produce theradiation pattern for the well known Tacan navigation systems have beennotably successful. Mechanical rotation of an array of parasiticallyexcited elements about a central exciting source has proven to be one ofthe simplest and most satisfactory methods for producing a space patternrequired for navigational aids such as Tacan. In the past, many suchantennas have utilized a single set of parasitic elements. This has ledto certain operating difficulties due to the fact that operation isoften required over two relatively Widely separated discrete frequencybands. One set of parasitic elements which may be satisfactory in one ofthe frequency bands does not operate efficiently in the other frequencyband. The prior art has attempted to remedy this situation by providingtwo sets of parasites, one for use at each frequency band for example.This technique has been only partially successful. At either frequencyband the presence of the parasites which are intended to Work at theother frequency band has led to considerable distortion in the radiationpattern. Thus, multiple sets of parasites interfere in the frequencybands where they were not intended to be used.

In other prior art antenna systems which are entirely stationary, it isoften desirable yto be able to change the shape of the antenna patternat will for various purposes, such as beam shaping or signalling. Theprior art methods have often involved clumsy expedients, such asmechanical linkages and motions, to relocate the parasitic elements andin some case necessitating reassembly .by hand so that the antenna maynot be used at all times. The present invention is intended to overcomethese difficulties and to provide other beneficial results as well.

It is 'another object of this invention to provide an antenna systemwhich will operate with extremely low distortion over two discretewidely separated frequency bands.

f provide impedance elements which are coupled to a parasitic element tocontrol the parasitic element.

It is still another feature of the present invention to provide anantenna with several distinct groups of para- 3,I36,996 Patented June 9,1964 sitic elements and several distinct groups of impedance elements,one impedance element connected to each of the parasitic elements. Bychanging the frequency of operation of the central source, the impedanceelements connected to the parasitic elements can be made to couple inorcouple out Various members of the parasitic element groups.

It is another feature of my invention to provide a plurality ofparasitic elements eachphaving an associated impedance determining unitso arranged that the parasitic elements may be readily rotated at highspeed without complications from the necessity for directly controllingthe impedance terminating devices.

The above-mentioned and other features and objects of this inventionwill become more apparent by reference to the following descriptiontaken in conjunction with the accompanying drawings, in which:

FIG.,l illustrates the nature of the frequency spectrum for which thepresent invention is particularly suited;

FIG. 2 is a perspective view of one embodiment of my invention;

FIG. 3 is a perspective View of another embodiment of the invention; i

FIG. 4 is a plan View from the top of the embodiment of FIG. 3;

FIG. 5 is a side elevation View of FIG. 4, taken along lines 5 5.

FIG. 6 is a plan View of a third embodiment of the invention;

FIG. 7 is a side elevational view of the embodiment of FIG. 6, takenalong lines 7-'7.

Referring now to FIG. 1, there is shown a plot of a two-band frequencyspectrum, that is, there are two frequency bands of interest-a lowfrequency band centered about some frequency such as F1 and alhighfrequency band separated by a certain frequency spectrum from the lowfrequency F1. The high frequency band is centered about the frequencyF2. The present invention is particularly suited for providing parasiticelements which can be made to operate over one frequency band but whichwill not interfere at all with the operation of the antenna over otherdifferent frequency bands. The present invention is also applicable overmore than two frequency bands, such as 3, 4, 5 frequency bands, etc. aswill be apparent from the explanation below. In the case of Tacan-typeantennas, the lower frequency band centered about frequency F l hasapproximately a 6 percent bandwidth. The lower frequency F1 isapproximately 980 megacycles per second. The high frequencyv band alsohas a bandwith of 'approximately 6 percent and is centered about a highfrequency F2 of approximately 11857megacycles per second. The twofrequency bands, measuring from center of the band to center of theband, that is, from frequency FlV to frequency F2, are separated byapproximately an 18 percent change in frequency. There is thus afrequency band of approximately l2 percent between the two bands whichis used for other purposes and in the case of Tacan, the l2 percentfrequency spectrum is used for distance measuring equipment Vbut itmakes no difference what particular use is made of the frequenciesbetween the useful bands. The lower limit of the low frequency band, andthe upper limit of the high frequency band are separated` by anapproximately 25l percent change in frequency. Itis more useful andconvenient to express the separations of the frequency bands inpercentage because this is what is useful in the calculation of theappropriate type of impedance element to cooperate with the parasites.It should be understood that our method is not limited to the particularexample we show in the specification but may be used to providecontrollable parasites which are useful over any two separated frequencybands or even over two discrete tric structure 5. The dielectric support5, could be aV sheet ora pedestal of dielectric material.V High-bandparasite 3 consists of two equal portions, 3A and 3B. The lengthof theparasite 3A is shown as and this is also the length of the parasite 3B;The two portions of the high-band parasite 3A and 3B are co-l supportmember 5. The length of either member 4A or line 6 is connected to oneportion 3A of the high-band parasite 3 at 7 as shown. The otherconductor 6B of the transmission line 6 Vis' coupled to the otherportion 3B lof the high-band parasite 3 at terminal 8 as shown. Thetransmission line 6 is terminated ina short circuit 9 which connects theone conductor 6A to the other conductorV 6B. The length of thetransmission line 6 is shown as L2. Transmission line V6 lies in theplane of the suptral radiator 2. The central radiator 2 then radiatesthe energy outward toward the parasitic elements 3 and 4. It should beunderstood that the particular frequency' used may be located at anyVVdiscrete frequency within the lower frequency band centered about Fl forexample. It need not be exactly at the frequency Fl as long as itisfwithin the calculated limits of the lower frequency band. The energyfrom the central radiator'Z strikes both parasites 3 and 4. However,only one of the two parasites will reradiate the energy and modify the Yradiation pattern. The other parasiterwill have no effect. The low-bandparasiter4 is responsiveto the radiation from the central radiator 2 atthe low band of fre-y quencies and the low-band parasite 4 willreradiate this energy. Thelength LP is appropriately chosen te beanefficient radiator of energy at the low frequency band. The length LPr'should be approximately one-*half Wavelength.V However, due to fringingeffects and end effects, an actualpractical4 length of V.V633 wavelengthis used for LP for the'low frequency band centered aboutFl;V

For yoperation in the low frequency band, the length L1' of thetransmission 10 has been appropriately chosen so that at the 10W bandfrequencies, the transmission line 10 appears to be, asiseenfrom theterminals 11 and 12, a

short circuit. For example,'it iswell'known vthat a onehalf wavelengthtransmission line which is shorted appears to be a short circuit asviewed from its terminals. Thus,

the length L1 of the transmission line 10 is appropriately chosen interms of the wavelengths of the low frequency bands centered about F1 toappear to be a short circuit as seen at the terminals .11'V and 12. Theshort circuit provided by the transmission line 10 effectively connectstogether the terminals 11 and y12 of the parasitic element 4. Thus,theqparasitic elements 4A` and 4B act togetherV asl one effectiveparasitic element Vof length LP and operating within the low frequencyband. The parasitic'elements 3 and 4 may be made of either dielectricmaterial or conductive material. When the parasitic element 4 iseffectively reradiating it will produce a peak'in the radia` port member5 and is located on aradial line running from the central radiator 2'vthrough the high-band parasite 3. The high-band parasite 3 is located aradial distance R1 from the central radiator 2 as shown. Anothertransmission line10 is connected to the low-band parasite 4. Oneconductor 10A of the transmission line 10 is connected to one portion 4Aof the 10W-band parasite 4 at terminal 11. The other conductor 10B ofthe transmission line 10 is connected to the other yportion 4B of thelow-band parasite 4 at terminal 12. Tlheilength of the transmission line10 is shown as L1. Transmission line 10 is terminatedi-n a short circuit13 which connects theV one conductor 10A to the other conductor 10B. Thelowerband parasite 4 is located a radial distance R2 from the centralradiator 2. Transmission lines 6 and 10 maybe axial cable. Thetransmission line 10 is also located on v a radial straight line fromthe central radiator 2 passing through the low-band parasite 4. `Radiofrequency energy is provided to the .central radiator 2 by means ofthecoaxial cable 14 from the source `of RF energy 15. The source of RFenergy 15 is capable of providing RF radiation at either of two discretefrequency bands centered about two frequencies F1, ,the center of lowfrequency band, and F2,vthe center of the high frequency band. r

' In operation, energy within'a frequency band centered about Fl' isprovided by the source 15 through the coaxial cable 14 via switchingmeans 1V4arto the cention pattern whenthe oper-ation is within the' lowfrequency band.V To understand how operation is accomplished over twofrequency bands, we examine the behavior of the high-bandparasite 3duringV operation Vof the antenna the lowvfrequency bands centered aboutFl. Thehigh-band parasite 3 willbe producing virtually'noradiationvduring operation within 'the low Y frequency band. Tounderstand why this is so, note that the transmission line 6 of lengthVL2 will not act as an Y lineV 6 appear to be a short circuit atVterminals 7 and-3 when viewed from terminals 7K', 8 for Voperationwithin thehigh frequency band centered above frequency F2.

-Butfor operation within -the low frequency bandythe transmission'line 6is not the appropriate length iny terms of wavelengths ofthe highfrequency band to provide an effective short circuit. HAtthe lowfrequencyband, the Aimpedance looking in terminals 7 and 8 appearsto beVquite'highj andthe shortcircuit19'at they otherend ofthe transmissionline 6ldoes not provide an `effective RF, short j circuitasfseen at theterminalsj and 8.v Hence,- the high-(1 Y,

band parasite SisA- split Vinto two separate distinct portions,

3A Vand3B,rwhich are separated by a relatively high impedancefbetweeri`terminals 7 and '8, Thel high-band parasite 3 cannot act as an efficientone-half wavelength;

radiator -Iand virtually no radiationtakes-place frornthehigh-bandfparasite' 3for radiation within theelow fre'- quency band.4AThe high-bandnparasite 3 contributes vir- 'l highfrequency` bandcentered about the highfrequency y -the n` situationzisVV justreversed.l I V'l-`l1e source of tually no elect to Vreradiation and hasvery little eect Vupon the' space pattern ofthe antenna-lfm' Yoperation'withinthe low frequency band. j t n A v r- A. However,.foroperation ofthe antenna 1 within .the

energy provides the central radiator 2 with a frequency Within the highfrequency band centered about F2, Highband parasite 3 is now the primaryreradiator of energy and the low-band parasite 4 has virtually no effectupon the radiation pattern when operation is within the high frequencyband. This is because transmission line 6 for operation at the highfrequency band now has the appropriate length in terms of wavelengths toprovide an effective short circuit as viewed from the terminals 7 and 8so that the parasitic element 3 is an effective single coni' tinuousradiator. The high-band parasite 3 appears effectively as one completeunit because the relatively low impedance between terminals 7 and 8effectively joins the Y two portions 3A and 3B into one continuousradiator of the appropriate length near one-half Wavelength. Forpractical purposes mentioned before, the actual length of LP willnormally be .633 wavelength as calculated at the high frequency band.Likewise, for operation Within the high frequency band, the low-bandparasite 4 appears to be two isolated separately distinct portions ofradiator 4A and 4B, separated by a relatively high impedance seen atterminals 11 and 12. The impedance seen at terminals 11 and 12 is nolonger a short but is instead a high impedance because the length L1 ofthe delay line 10 is no longer correct to provide an effective shortcircuit at the wavelength within the high frequency bands.

Thus, it can be seen that for operation at either frequency bands, onlyone of the two parasites 3 or 4 is effective at any one time for shapingthe radiation pattern of the antenna. In effect, the transmission lines6 or 10 remove their respective parasites when the operation is outsideof the frequency band for which the parasite was intended. All that isnecessary to eliminate the effect of the particular parasite is tooperate the energy source 15 at a frequency outside of the frequencyband for which the parasite and the delay of the transmission lines 6 or10 was designed. An example will show that the method and techniqueillustrated by the embodiment of FIG. 2, can be used wherever an antennais to operate over a multiplicity of frequency bands and it is desiredto have an appropriate radiation pattern for each particular frequency.In general,

where A is the wavelength corresponding to the frequency F and V isequal to the velocity of propagation of the medium which is used. In'thecase of electromagnetic radiation, V is equal to the speed of light andis approximately 3.0 l() meters per second.

V (2) M- lowpband (3) xa-Y- hh b d -F2 1g an Suppose we wish to designan antenna with two sets of parasites to operate over two discretefrequency bands, onel frequency band being centered about the frequencyFl. For convenience let us call this the low frequency band. Thus, thewavelength of radiation at the low band is Al and the wavelength ofradiation at the high frequency band is A2. We can also terminate theparticular transmission line in either a short circuit or an opencircuit. A short circuit termination Was illustrated in FIG. 2 and forthe purposes of discussion we will use a short circuit termination also.It is well known that if at some particular Vpoint along a transmissionline the impedance at that point appears to be a short circuit, onequarter of a wavelength away the impedance will appear to be a maximum.Then one-half wavelength away from the original point, the impedanceappears to be a minimum again and so on with maxima and minima ofimpedance alternating down the length of the transmission line. It isthen clear what must be accomplished when the operation changes fromwithin the frequency band near F l to a frequency within the band nearF2. `The particular percentage change in frequency between the twofrequen-l cy yhands must result in an equivalent 90 phase shift in thetransmission line, that is, the effective length of the transmissionline must go through approximately a one-quarter wavelength change or,in other words, a 90 phase change in effective length of thetransmission line. This will cause the transmission line to look like ashort circuit as used from its own terminals Vat its own frequency band.But when operation is changed to the other frequency band, the impedanceat the terminals of the transmission line will appear to be very highdue to the fact that in terms of the new frequency of operation at theother frequency band the effective length of the transmission line hasbeen changed because of the change in wavelength by the change infrequency.

We can write this general relationship as Equation 4.

where Ao is the required phase change for switching from one frequencyband to the other and AF represents the percentage change in frequencyof the two bands as compared to one of them. From the discussionimmediately above, it can be seen that Arp must be equal to 90electrical degrees, that is, the effect of one quarter of a Wavelengthchange in length of transmission line between the two bands. AF ingeneral will be determined bythe particular requirements of thecommunication system with which the antenna has to work. In the case ofthe Tacan type of equipment, the approximate calculations can beperformed as follows. Let Fl be equal to 980 megacycles. Let F2 be equalto 1185 megacycles. This is illust'rated in FIG. l.

AF is equal to ll minus980 divided by 1185:

Ll is the required length of the transmission line for use at the lowfrequency band. Thus, AF equals .18 or an 18 percent change in frequencywhen going from one band to the other. For convenience, all thecalculations will be performed based upon the higher frequency of 1185as the reference. Now using the value of AF of 18 percent and the valueAq of electrical degrees, we ,can calculate L1 in terms of electricaldegrees at the frequency of Fl the low frequency band.

M (6) L1-AF---l8 degrees (6A) L1=499 degrees The length of Ll is equalto 499 electrical degrees. To calculate this in terms of Wavelength, wesimply divide by 360 degrees which represents one wavelength at thefrequency F l.

So the choice of the nearest half-wavelength line must be made. It maybe convenient tol choose one-half wavelength as the length of thetransmission line because a shorter length of transmission line will bemore suitable for physical mounting and packaging arrangements. However,the advantageous length that'must be chosen is 1.5 wavelengths becauseas the calculation shown by Equation 6 indicates, at least l.388wavelengths are needed to produce the appropriate amount of change inelectrical degrees per unit change in frequency. lIt. will be understoodthat although as far as producing a minimum impedance is concerned, aone-half Wavelength line and Wavelength or 1.5

a 1.5 wavelength line are equivalent, the, two lines are not equivalentin the total amount of phase delay or phase change which they producebecause the electrical phase change is'cumulative and continuous downthe length of the line so that a 1.5 wavelength line produces threetimes as much total delay or 3 times as much total'phase change as'I aone-half wavelength line does. Therefore, L1 will be chosen as 1.5wavelengths at the lower band frequency. To actually calculate thislength, we perform the followlng:

V (7) Y L1 1.5 \1 1.5

3X 108 meters (2B) masoxioaps (7A) L1=1.5 30.6 om.

k1 the wavelength at the low frequency band is simply V/Fl or 30.6centimeters.

(7B) L1=45.9 em.

' for transmission line 10. (7A) Vshows that L1 :1.5 30.6

centimeters or 45.9 centimeters'for the length of Ll.

. AThe length L2 for the transmission line 6 of high frequency bandparasite 3 may now be also readily calculated. It can be seen that thepercentage change in frequency will be again 18 percent as shown byEquation 5 because the change in frequency is the same whether movingfrom the lowY band to the high band or from the high band back tothe lowband. Again for the 90 phase change, a transmission line of lengthl.5)\2 is required; The actual physical length of this line is diferentfrom the length of Ll because a wavelength is physically diierent at thehigher frequency band. To cal-y culate the length of line 6 rstcalculate X2 from Equation 3 and Equation 3B 3X 108 meters 11.85 108c.p.s.

A2 is equal to 25.3 centimeters. Hence, (8) L2=l.5 }\2=l.5 25.3centimeters L2 is equal to 37.95 centimeters. tion, we can take the tworatios of L1 45.9 (9) '-nzg-l-LSI In Equation 10 which is Anotherembodiment ofthe invent1on is shown in FIG.

3. The above method of calculation can be used for any two separatedfrequency bands-to calculate theV appropriate length for transmissionlines to accomplish the irnpedance matching of the parasites when thefrequency is changed. FIG. 3 shows an antenna 16 with a centralradiating unit 17. The antenna ofFIG. 3 is constructed, however, toprovide for mechanical rotation of the parasites aboutthe centralradiating unit 17. kThe central radiating unitf17 is composed, of twoinner stationary radiating mein-bersV 18 and 19. The two members 18 and19 may be Vin the shape of hollow cylindrical cups. The

two stationary radiating members 18 and 19 are separatf ed byanfnsulating element 20. RF energyV isprovided transmission lines whichare yet to be described form a i 8 to the central radiating unit 17through the'coaxial cable `21. The outer conductor 22 is connected tothe lower radiating cup- A19. Therinner conductor 23 is connected totheupperrradiating cup 18 by being passed through a suitable'hole'through the center of the insulating member 20. Thus, the centralradiating unit 17 acts somewhat like a `dipole and provides anYomnidirectional pattern of. RF energy-torbe radiated by the antenna.vMounted for rotation about ,theV central radiating'members 18 `and 19is a dielectric cylinderr24. A supporting sheet or plate of Y Y Theouter cylinders 26 and 27 are concentric with therdi-` electric`cylinder 24;` and` the cylinders 24, 26 and 27 and the dielectricsupport plate 25 Yrotateras a complete unit together. A` suitablebearing 28, gears 29 and a motor 30 provide power to rotate the antennaassembly. A fundamental parasite 31 made of a strip of conductor-or avblock of dielectric'is mounted on` or'fwithin lthe Vinner cylinder24/and`this provides the modulation of the funda; mental frequency forabearing facility antenna,. suchas Tacanpl [i A-lrighk frequency bandparasite 32 ismounted on or within the" middle cylinder V26 arid a lowfrequencyband parasite 33 is mounted within or lon the outer cylinder27.` A top plan view of the antenna of FIG. 3 is shown in FIG. 4.and aside elevational view of the same antenna is shown in FIG. 5. The middlecylinder 26 islocated at a radius VR3* from the central radiator 17. Theouter cylinder 21. is located at a radius R4 fromthe central radiator17. The group of Vtwo parasites Vsuch Vas 32 and 33 with the associatedcylinder walls and associated functional group'and as seen in FIG. 4,there are actually nine' such groups equally spaced on the antenna.There is an 'angular` spacing of 40 between the'groups of two parasites.The other groups of parasites are each gen-` erally designated aselements 34A, 34, 35, 36, 37, 38,

39, 40 and 41. The groups areidentical in construction and vfunction anda description of one group will suice To check the calcula- Y to makeclear the operation of the antenna. The high frequency parasite 32 hastwo equal coaxial portions,

32A and 32B located in vertical alignment as shown. The length of theparasitic element 32B is given'as and the two halves oftheparasiticelements'SZA and`32B are of equal length. L5 is chosen` to bean appropriate length to provide an eilicient dipole radiator andreradiator `at the high frequency band. The calculations will beexplained below. Likewise, the low-band parasiteY 33 has two equallengthcoaxial portions 33A and 33B vertically aligned as shown. n Thelength 'of the parasitic element 33A or 33B is A l L6 Y and the lengthL6 is chosen to be an eicient dipolere-KV radiator at the low frequencyband.

Attached to the high frequency parasite 32 isa trans-V missionV line 42.The top conductor 43.0f transmission line 42v is connected to one end ofthe parasitic element 32A at terminal 44. The other conductor 45 of thetransmission. line 42 is connected to the parasitic ele.- rnent 32B atthe Vterminal 46. The two conductors 43 and 45 of the transmission line42.*are spaced -apartby the dielectric Vsupport plate 2S. In FIG. 5,the'vertical dimensions of the transmission line 42 have been somewhatexaggerated for clarity of illustration. The trans# mission line 42'might Vtypically consist of a thin, printed, Ystrip of conductor on-the surfaceof the plate 25. However, a two-wire transmission line maybe used or a con-` ductor embedded inpapblock of dielectric material maybe used at 43 with equal facility. The parasitic elements 32 and 33 havebeen shown embedded within the walls of the dielectric cylinders 26 and27 for secure physical support during rotation. However, the parasites32 and 33 could be mounted on the surface of the cylinders. The lengthof the transmission line 42 is L4. The length L4 of the transmissionline 42 for high-band parasite 32 is calculated in the same manner asthe length L2 of the high-band transmission line 6 for the parasite 3 asshown in FIG. 2 and explained above. For a 20 percent change infrequency in going from the high frequency band to the low frequencyband, the length L4 will typically be 1.5 wavelengths at the highfrequency wavelength. Again, the transmission line 42 acting incooperation with the parasitic element 32 provides a frequencyresponsive device which effectively uncouples the parasite 32 from thecentral radiator at the low frequency band and the transmission line 42effectively allows the parasite 32 to be coupled to the central radiator17 for operation within the high frequency band. This operation is aswas explained in conjunction with FIG. 3.

The low frequency band parasite 33 has coupled to it anothertransmission line 47. The transmission line 47 is formed from a topconductor 4S and a lower conductor 49. The two conductors 48 and 49 asbefore are mounted on each side of the support plate 25. However, theconstruction of the transmission line 47 oifers a number of uniqueadvantages. The one conductor 4S of the transmission line 47 is formedin four parts, 43A, 48B, 48C and 43D. The portion 43C is connected tothe parasitic element 33A at terminal 50. The conductor 43C thenbranches into a cross connecting piece 4SD. The connecting portion 48Dconnects together two straight parallel conductor portions 48A and 48B.48A is located parallel to the transmission line 42 and on one side ofit. Likewise, the conductor 48B is located on the other side of thetransmission line 42 and parallel to it. Portions 48A and 48B of thetransmission line 47 pass through ythe wall of the middle cylinder 26 bysuitable apertures therethrough. This is indicated at FIGS. 4 and 5. Forclarity, cylinder 26 has been omitted from FIG. 3. The lower conductor49 of the transmission line 47 likewise is composed of four portions,49A, 49B, 49C, 49D. The respective portions A, B, C, D, of the conductor49 lie directly under and on the opposite side of the dielectric plate25. The respective portions of conductor 49 correspond and are similarlylocated tothe portions A, B, C, D, of the top conductor 4S. Thus, thetransmission Vline 47 is parallel to and brackets transmission line 42.

This type of construction has several important advantages. First, thereis no possibility of RFinterference between the two transmission lines42 and 47 because the two equal portions 43A and 48B of the transmissionline 47 which bracket the transmission line 42 are parallel andequidistant from the transmission line 42 and all portions of thetransmission line 47 carry the same current. Fluxy from the portion 48Aof the transmission line 47 vwill cancel flux due to the portion 48B ofthe transmission line 47, and there will be no possibility ofelectromagnetic coupling with the transmission line 42, This allows thehigh-band and low-band parasites 32 and 33 to be located in line witheach other along the same radial line as shown in FIGS. 3 and 4.Locating the parasites 32 and 33 in radial alignment has the advantagethat only one set'of reference disks (not shown) are required to be usedwith such an antenna. @ne particular group, for example group 34 ispicked as the reference group. Each time the group 34 passes a certainreference direction, for example north, the antenna is required to sendout a reference signal. This is usually accomplished by having areference disk with metal slugs or other devices mounted on it. Thisreference disk rotates in iixed relationship with the antenna assembly24, 2S, 26, 27. Thus, when the two sets of parasites are radiallyaligned, operation at the two frequency bands may be accomlt) plishedwith the use of only one reference disk to create the reference pulsessince the azimuth relationship of the two sets of parasites will be thesame.

The length of the transmission line- 47 is shown as L3 and L3 is chosento provide a low impedance match for low-band parasite 33 for operationin the low band. The transmission line 47 will appear as a largeimpedance for operation at the high frequency band. Thus, transmissionline 47 acts as a frequency sensitive device which alternately couplesand uncouples the parasite 33 to the central radiator 17 depending uponwhich frequency band is used. The parasitic element 33B is connected tothe transmission line conductor 49C at terminal 51.

The total length L3 of transmission line 48 (or 47) can be written aswhere the subscripts refer to the four branches of the transmission line48. The branches LA and are in series, and branches LB and The twoportions are in series.

and

LD (Lets and v l LD (Liri-** in series with portion LC only lowers themagnitude of the maximum impedance of the line and does not change thephase shift of the line., v

Hence L3 is calculated asv explained before in connection with length L3of FIG. 2. The portions LA, LB, (LA=LB), LC and LD can be chosen forconvenience as long as the total length given by Equation 11 or 12 forL3 is satisiied. Thus for a particularvalue of L3 the lengths LB, LA, LCand LD can be chosen from a large range of combinations determined bythe physical size and the operating frequency, etc. of the particularantenna in question.

It will be obvious to those skilled in the art that any convenientnumber of groups of parasitic elements could have been used. Thus, theremight have been one group as was shown in the stationary antenna of FIG.3 or there might be 2, 3, 4, etc., or any number of parasitic elementsconveniently arranged in bearing on the antenna. Likewise, the antennaof FIGS. 3, 4 and 5 need not be rotated if such rotation is not requiredfor the particular type of radiation pattern which it is desired tocreate. But the provision of two sets of parasites allows operation tooccur over two distinct frequency bands Without any further adjustmentsand without stopping the operation of the antenna. It was pointed outthat the high frequency band elements are located at a particular radiusR3 from. the central radiator; that the low-band parasites, such as 33,at a radius ofA R4 from the centralk radiator` 17. The radii R3 and R4are of critical importance in providing an eicient modulation of the'radiation pattern. In my United States Patent No. 2,928,087 issuedMarch 8, 1960, I have shown in some detail the mathematical basis forthe choice of the radii R3 and R4 and the effect of this radii on theelevational pattern. FIGS. 5 and 7 in particular emphasize theimportance of a proper selection of the radius R3 or R4. A,method ofcalculating the radius R3 and R4 does not form any part of the presentinvention. However, -the present invention is novel in that by allowingthe use of two separate sets of parasites, one for each frequency band,the two sets of parasites may each be located at different radii asshown so that the radius of each group of parasites may be picked at theparticular optimum physical radius which is desirable for operation atthat particular frequency band corresponding to'the band of thatparticular set of parasites. Optimum modulation of the radiation patterncan be achieved at both frequency bands by the use of two sets ofparasites which can be coupled in and coupled out as Y shown by thepresent invention.

The length L5 or the length L6 is chosen so thatl the particularparasite will be an efficient radiator at its own wavelength. Thus, L5would be chosen to be approximately one-half Wavelength at the highfrequency band. In other words, L5 will be .5 times A2 in wavelength.However, as was indicated, because of practical considerations offringing, usually a length of .633 of a Wavelength will be used. L6 islikewise made .633 of a wavelength at the low frequency band so that L6is equal to .633 times kl. A continuous half Wavelength type radiator isa very efficient reradiator of transmitted energy. However, when theparasite is split into twov equal portions which are each approximatelya quarter of a wavelength long or .31 wavelength long then the impedanceofV the two separate portions is quite high and they do not form aneffective radiator. This is the reason why the change in impedance ofthe transmission line can act as a means for effectively coupling in andout the parasites with the two different frequency bands. The use of thetransmission line effectively makes the undesired group of parasitesdisappear at the undesired frequency band.

FIG. 6 and FIG. 7 show another type of construction which maybe usedutilizing the principle of the present Y invention. Corresponding partsare numbered with cor-l There are provided a set of low frequency bandparasitic elements 52 and a set of high frequency band parasites shownas 53. The parasites 52 and 53 may be made of conductors or ofdielectric material. As before, a set of central portion of the low-bandparasites 52. Likewise,

' there is a high frequency transmission line y55 coupled to the centralportion of each'of the high-band parasites 53. The low frequencytransmission line 54 has a length L1 calculated in theV same manner asthe example shown in Y connection with FIG;V 2. The high'frequencytransmission linev 55 has alength L2 calculated in the same mannerasthelength L2 shown in connection'with the'examp-le A,of FIG. 2.3 Theftransmission line for the low-band para-.

vides for fundamental modulation of the radiation pattern. GQ i lowfrequency transmission lines 54 are connected to the 65 site 54 passesthrough the wall of the cylinder 26 by means of a suitable slot oraperture. As before, the highfrequency parasites 53 are effective inchanging the modulation pattern of the antenna only for operation at thehigh frequencies and the transmissionl line 55 effectively causes thehigh band parasites to stop radiating for operation in the low frequencybands. Likewise, the low frequency parasites 52are effective onlythrough operation in the low frequency band and the transmission lines54 cause parasite 52 to become an ineffective radiatorfor operation atthe high frequency bands so that low frequency parasites 52 have littleor no effect on the modulation pattern for operation at the highfrequency band.

However, the antenna of FIGS. 6 and 7 has considerable advantages in theconstruction shown.- First, a dilferent number of high parasites may beused compared to the low frequency parasites. For example, nine lowfrequency parasites, such as 52,y may be used but any'ditferent numberhigh frequency parasites-53 may be, used. This. is because transmissionlines 55 and 54 of this antenna are not interrelated as Wasthe case inFIG. 3 and FIG. 4.' operation at onefrequency band, the other set foroperation at the other frequency band, thev high Vfrequency band forexample will be modulated by the 'number of parasites in the highfrequency band. For example,.nine

is shown. However, iffor example six high frequency parasites 53 wereused for operation at the high frequency band, modulation will be asixth harmonic modulation rather Vthan a ninth harmonic modulation whichwould occur at the low frequency band. Thus, as before,trans missionlines 54 and 53 Will remove the effect of the other set of parasitesforoperation during either frequency band. Thus, the antenna of FIGS. 6and 7 provides a tern of the antenna' if a ditferentgnumber of parasitesis used at 4the two frequencyv bands.

While I haverdescribed above'the principles'of my invention inconnection with specific apparatus, it'is to be clearly understoodthatthis description is made by way of illumination and not as alimitationto the scope of thereof and in 1. An omnirange beacon antennacomprising a centralV 'radiating element, a first group of parasiticreradiators adapted to operate at a first frequency, a first group ofVyparasitic reradiators, adapted to operate at a secondv `frequency, asecondV group of transmission lines, each of saidV second group oftransmission lines coupled to-each parasitic reradiator of saidsecondgrouplof parasitic reradiators, each of said first groupoftransmission lines having a length different from each of saidfsecondgroup of-transmission lines, Asaid centrale-radiatingelement-transmitting electromagnetic radiation at either saidfirst'frequency or said second frequency as desired, whereby said rsrtgroupV of reradiators Will affect the radiation Vpattern ofvsa'idcentral radiating element'at said rst frequency and said second'group ofreradiators will alect the said radiation pattern at said secondfrequency.v i

2. An omnirangebeacon antenna according to Yclaimwl` i LAwherein each ofsaid parasitic reradiators consists' of a first portion-and a secondportion whose lengths are'equal to eachother, the lengthgof therespectivel portions of. k each o f said parasitic reradiators of saidfirst group being different from the length of the'portions of eachparasitic reradiator of said, second group,each of said transmission Iftwo sets of parasites are used, one set for 13 lines having a shortcircuit at its end remote from said parasitic reradiators.

3. An omnirange beacon antenna according to claim 2 further comprisingmeans for supporting said transmission lines and said parasiticreradiators, and means disposing said transmission lines and saidparasitic reradiators for rotation about said central radiating element.

4. An antenna according to claim 3 wherein the number of parasiticreradiators in said first group is different from the number ofparasitic reradiators in said second group.

5. An omnirange beacon antenna according to claim 4 wherein said firstportion and said second portion of each said parasitic reradiator arealigned vertically on an axis with each other, each of said first groupof parasitic elements being disposed'at a first radius from said centralradiating element, each parasitic reradiator of said second group ofparasitic reradiators being disposed at a second radius from saidcentral radiating element.

6. An omnirange beacon antenna according to claim 5 wherein the lengthof each of said first group of transmission lines is substantially 1.5wavelengths at said first frequency, the length of each of saidtransmission lines of said second group of transmission lines issubstantially 1.5 wavelengths at said second frequency, the combinedlength of the two portions each of said parasitic reradiators of saidrst group is substantially .633 Wavelength at said first frequency andthe combined lengths of the two portions of each parasitic reradiator ofsaid second group is substantially .633 wavelength at said secondfrequency.

7. Apparatus for controlling modulation for use in an antenna having acentral radiating element, comprising a plurality of parasitic elements,a plurality of impedance elements, each one of said plurality ofimpedance elements being coupled to a corresponding onev of saidplurality of parasitic elements, each of said impedance elements beingresponsive to a predetermined frequency transmitted by said centralradiating element whereby said parasitic elements reradiate energy onlyat said predetermined frequency.

8. An antenna comprising a central radiating element,

a first parasitic reradiator adapted to operate at a first frequency,

a first transmission line connected to said first reradiator,

a second parasitic reradiator adapted to operate at a second frequency,

a second transmission line connected to said second reradiator, thelength of said first transmission line being different from the lengthof said second transmission line,

the length of said first parasitic reradiator` being different from thelength of said second parasitic reradiator, Y Y

said central radiating element vtransmitting electromagnetic radiationat either said first frequency or said second frequency as desired,whereby said first reradiator will affect the radiation pattern of saidcentral radiating element at said first frequency only and said secondreradiator will affect the said radiation pattern at said secondfrequency only.

9. An antenna comprising a central radiating element,

a first parasitic reradiator adapted to operate at a first frequency,

a first transmission line connected to said first parasitic reradiator,

a second parasitic reradiator adapted to operate at a second frequency,

a second transmission line connected to said second parasitic reradiatorsaid second transmission line consisting of two conductors disposedparallel to each other,

each of said two conductors having two parallel pori4 Y tions of equallength, one portion disposed parallel to and on each side of said firsttransmission line, said first transmission line having a lengthdifferent from said second transmission line,

said central radiating element transmitting electromagnetic radiation ateither said first frequency or said second frequency as desired,

whereby said first reradiator will affect the radiation pattern of saidcentral element at said first frequency and said second reradiator willaffect the said radiation pattern at said second frequency.

10. An omnirange beacon antenna transmitting radio frequency energy at afirst frequency and at a second frequency as desired comprising a firstparasitic reradiator, adapted to operate at said first frequency, thelength of said first reradiator being proportional to the Wavelength atsaid first frequency,

a first transmission line connected to said first reradiator, a secondparasitic reradiator adapted toy operate at said second frequency, thelength of said second reradiator being proportional tothe wavelength atsaid second frequency,

a second transmission line coupled to said second reradiator, said firsttransmission line having a length different from said secondtransmission line,

whereby said first reradiator will affect the radiation pattern of saidcentral radiating element only at said first frequency and said secondreradiator will affect the said radiation pattern only .at said secondfrequency.

ll. An antenna according to claim 10 wherein both said first and saidsecond reradiators are disposed along the same line passing through saidcentral radiating element. f

l2. An antenna according to claim l0 wherein said second transmissionline consists of two conductors disposed parallel to each other, each ofsaid two conductors having two parallel portions of equal length, oneportion disposed parallel to and on each side of said first transmissionline. i

13. An antenna comprising a central radiator,

a first parasitic reradiator tuned to a first frequency,

a second parasitic reradiator tuned to a second frequency,

impedance elements connected to said parasitic reradia tors and`responsive to said first and lsaidsecond frequencies so that when saidcentral radiator transmits radio frequency energy at said firstfrequency, said impedance elements act to cause said first reradiator toreradiate a maximum of said energy and said impedance elements act tocause said second reradiator to radiate substantially no energy.

14. An antenna comprising a central radiator,

a first parasitic reradiator tuned to a first frequency,

a .second parasitic reradiator tuned to a second frequency,

impedance elements connected to said parasitic'reradiators andresponsive to said first and said second frequencies so that when saidcentral radiator transmits radio frequency energy at said firstfrequency said impedance elements act to couple said first reradiatorand to uncouple said second reradiator.

l5. An antenna comprising a central radiator,

a first parasitic reradiator adapted to operate at a first frequency,

a second parasitic reradiator adapted to operate at a second frequency,

first impedance element connected to said first parasitic reradiator andresponsive to said first and second frequencies, A I

second impedance element connected to said second parasitic reradiatorand responsive to said first and second frequencies so that when saidcentral radiator transmits radio frequency energy at said firstfrequency, said first impedance element acts to cause 16. An antennatransmititng radio frequency energy j comprising Y' a central radiatortransmitting radio frequency energy at two predetermined frequencies, vY a parasitic reradiator adapted to operate at a first of saidpredetermined frequencies, a rst transmission line connected to saidfirst parasitic reradiator and responsive to both said predeterminedfrequencies, .A

a second parasitic reradiator adaptedtofoperate at a` secondone of saidpredetermined frequencies,

a second transmission line responsive to both said predeterminedfrequenciesconnecte'd to saidrsecond parasitic radiator, said rsttransmission line having a length different fromV said secondtransmission line so that said first reradiator radiates a maximum ofenergy at said first frequency and said second reradiator radiatessubstantially no nergy at said first frequency and so that said secondreradiator radiates a maximum of energy at said second frequency andsaid first reradiator radiates substantially no energy at said secondfrequency. Y v

17. An antenna according to claim 16 wherein said first reradiator islocated at a radial distance Vfrom said central radiator which isdifferent from that of Vsaid second reradiator.

18. An antenna according to claim 17 wherein said first reradiator andsaid second reradiator are disposed on the same radial line passingthrough said central radiator.

19. An antenna comprising'a central radiator,

a first parasitic reradiator tuned to a first frequency,

a secondv parasitic reradiator tuned to a secondfrequency, l

a first transmission line connected to said first reradiator andresponsive to said rst and said second frequencies, Y Y

a second transmission line connected to said second reradiator andresponsive to said first and said second frequencies, d a l so that whensaid central radiator transmits radio frequency energyl at said firstfrequency said first transmission line actsto cause'said firstreradiator to radiate a maximum of said energy and saidsecondtransmission line acts to cause said second reradiator to radiatesubstantially no energy,

and so that when said central radiatortransmitsvradio said rsttransmission line acts to cause said first y a t reradiator tto radiatesubstantially no energy.

20. VAn antenna according to clairn 19 wherein said second transmissionline consists of two conductors dis,-VL

posed parallel to each other each of said two conductors having twoparallel portions of equal length one portion disposed parallel to andon each side of said first transmission line. Y Y 21. An antennaaccording to claim 20 wherein the length of said first reradiator isvdifferent from the length of said second reradiator. v .Y Y

22. An antenna transmitting radio frequency energy comprising a centralradiator transmitting radio frequency energy at two predeterminedfrequencies as desired, f, v a first parasitic reradiator having a firstportion and a second portion, v Y

said Y,reradiator to said second portion of said reradiator, saidtransmission line being responsive to a first frequency Aof said twopredetermined Vfrequencies, said transmission line being short circuitedr at one end,'so that said first transmission line presents a lowimpedance connected betwen said first portion and second portion of saidfirst reradiator at said 'first frequency and whereby said firsttransmissionY Y second frequency of said two predetermined fre-.vquencies, said second transmission linebeing shortV circuited at one endso that said second transmission line presents a low impedance connectedbetween said first portion and said second portion of saidv secondreradiator at said second frequency and whereby said second transmissionline presents a Vrelatively high impedanceconnected betwen said firstportion and said second portion of said second reradiator at said firstfrequency whereby the amount'of energy reradiated by said reradiators iscontrolled by said two predetermined frequencies.

first transmission line coupling said firstV portion of Y

7. APPARATUS FOR CONTROLLING MODULATION FOR USE IN AN ANTENNA HAVING ACENTRAL RADIATING ELEMENT, COMPRISING A PLURALITY OF PARASTIC ELEMENTS,A PLURALITY OF IMPEDANCE ELEMENTS, EACH ONE OF SAID PLURALITY OFIMPEDANCE ELEMENTS BEING COUPLED TO A CORRESPONDING ONE OF SAIDPLURALITY OF PARASTIC ELEMENTS, EACH OF SAID IMPEDANCE ELEMENTS BEINGRESPONSIVE TO A PREDETERMINED FREQUENCY TRANSMITTED BY SAID CENTRALRADIATING ELEMENT WHEREBY SAID